I have experienced a similar sudoers problem on my Synology NAS device.
My solution was to put the following command into the "Task Scheduler"
chmod 0755 /etc/sudoers
Which I set to run as a root.
Although my problem was permissions related, I bet a properly configured command line series of output redirections would clear out the file and replace the contents with a basic shell that would get you back up and running.
Something like:
echo "Defaults syslog=authpriv" > /etc/sudoers
echo "%administrators ALL=(ALL) ALL" >> /etc/sudoers
echo "Cmnd_Alias SHELL = /bin/ash, /bin/sh, /bin/bash" >> /etc/sudoers
echo "Cmnd_Alias SU = /usr/bin/su" >> /etc/sudoers
echo "%wheel ALL=(ALL) NOPASSWD: ALL, !SHELL, !SU" >> /etc/sudoers
sudoers
file from a backup.